Modern era Lionel GP-20 Chessie diesel #8463 motor problem?

Sarge,

I agree with anything published so far, but perhaps can add something. After you clean out the old grease (if in fact it is waxey or really solidly gunky) take a look at the top of the brushplate -- the plastic piece (quite possibly white) at the top of the motor.

At its highest point, directly above the top end of the shaft, there is a quite small screw with a recess for an Allen wrench, the L-shaped six-sided wrenchs that are often used in "set screws." This screw takes a very small wrench indeed.

The bottom of this screw protrudes downward through the brushplate and rests on the top of the rotating armature shaft. The tolerences here are critical. In his post above, Bob Nelson (lionelsoni)) has described what happens when this screw is mal-adjusted. Its primary purpose is to limit the upward travel of the main motor shaft. If the shaft moves either up or down too much, the gear on the bottom of the shaft won't mesh properly with the spur gear on the axle that drives the wheels; in other words, the gears will "jam," partially or even fully.

The action of the motor is such that when turning forward it tends to force the armature shaft in one direction (up or down -- I can't remember which) and when running in reverse the force is in the opposite direction.

By tightening or loosening this screw (just a hair is all it takes) you can go from a motor that may not run at all in one direction and will be "sloppy" in the other, to vice-versa. The goal, of course, is an adjustment where the shaft gear is centered on the spur gear, with only a very small amout of up or down "play" or "gear-lash." I think this is likely to be the source of your problem, and the only solution is to adjust it by turning the screw back and forth a little until a proper meshing of the gears is achieved. (I alway put a tiny dab of grease on the top of the shaft, because the screw make direct metal-to-metal contact at that point. One doesn't want to use so much grease as to have it run down onto the copper commutator face. When it runs equally well in forward and reverse you've got it. Properly adjusted, the screw does not eliminate vertical movement of the armature/shaft/gear, but it limits it to a few thousands of an inch -- just enough to let you feel and hear a slight click-click-click as you move the armature up and down. In a phrase: free but not sloppy.

Other things to check, are the cleanliness of the bush wells, the movement of the brushes within those wells, the condition of the brushes themselves, and the brush springs. If you decide to replace the brushes, replace the springs at the same time. If by any chance the set-screw threads in the brush-plate are stripped out, and/or the screw is missing, you will need new parts.

In my opinion, the 8030-100 Motor Truck was not Lionel's best design, but it can me made to work. The symptoms you describe are exacerbated by overloading the locomotive. Ten modern lightweight freight cars is about its limit. 

An exploded drawing is available at the site, under customer service. I don't see any thrust washers in that diagram. I think "the set screw is everything."